1. Field of the Invention
The present invention relates to a signal reproducing device and, in particular, to a signal reproducing device for forming a tracking error signal by detecting a specific frequency component in a signal reproduced from a recording medium.
2. Description of the Related Art
FIG. 1 is a diagram showing the construction of the control circuit of a digital VTR for performing reproduction tracking control by a hitherto proposed tracking control system using two kinds of pilot signals (hereinafter referred to as the "2f ATF system").
In FIG. 1, information recorded on a magnetic tape 1 is reproduced by two rotational magnetic heads 2a and 2b which are located on a drum (not shown) and have different azimuth angles.
FIG. 2 shows the recording pattern of the tape 1.
Tracks F0 to be traced by the head 2a and tracks F1 and F2 to be traced by the head 2b are alternately formed on the tape 1. Pilot signals whose frequency is f1 and pilot signals whose frequency is f2 are alternately recorded on the tracks F1 and F2 respectively. On the tracks F0, practically no pilot signals are recorded.
In FIG. 1, after having been amplified by reproduction amplifiers 3a and 3b, reproduction signals of the heads 2a and 2b are alternately extracted through a switch 4, which is changed over by switching pulses SWP, and output to an A/D converter 5 and an ATF circuit 6. The reproduction signals digitized by the A/D converter 5 are supplied to a video processing circuit on the output side.
FIG. 3 shows the frequency characteristics of a reproduction signal. As shown in the drawing, pilot signal components f1 and f2 are included in a reproduction video signal. The f1 and f2 components consist of those components which are obtained when the pilot signals of f1 and f2 components recorded on the tracks F1 and F2 are reproduced as crosstalk during reproduction on the adjacent tracks, the tracks F0, by the head 2a.
The reproduction signals output to the ATF circuit 6 are input to band pass filters (BPF) 7a and 7b, where the f1 and f2 components are extracted from them. The extracted f1 and f2 components are supplied to detection circuits 8a and 8b, where their respective levels are detected. Signals indicating the detected levels are averaged through low-pass filters (LPF) 9a and 9b, and then digitized by A/D converters 10a and 10b before they are supplied to a computing section 11.
The computing section 11 takes in the levels of the f1 and f2 components on the basis of the above-mentioned SWP pulses, and calculates the difference in level between them to thereby output an ATF error signal indicating the offset amount of the head and track. This ATF signal is added to a capstan servo circuit on the output side, whereby the phase of the head and track is controlled. The polarity of this AFT error signal is inverted for each track by the switching pulses SWP.
In the above-described prior-art example, shown in FIG. 1, the ATF circuit 6 for obtaining off-track amount is formed by two circuit systems in order to extract the frequency components f1 and f2 of the pilot signals recorded on the tracks F1 and F2 adjacent to the tracks F0 on either side thereof. These two circuit systems continue to operate from the instant of turning ON to that of turning OFF of the power source of the VTR, with the result that the following problems occur:
(1) The ATF circuit 6 is allowed to operate even during a period in which there is no need to obtain the off-track amount, for example, during the period in which the head 2b traces the tracks F1 and F2, resulting in a waste of electric power.
(2) There is no difference in accuracy in the requisite off-track amount and responsiveness between the dubbing mode, insert mode and normal reproduction mode, which means no optimization is attained.